Semiconductor diode lasers provide an intense efficient source of laser radiation. Continuous Wave (cw) semiconductor lasers have achieved output power of several watts by using either a broad-area or an array geometry. Such devices generally emit their radiation in a broad twin-lobed far-field beam in the direction parallel to the active layer of the laser. This broad twin-lobed output beam limits the ability of these laser sources to be focused tightly or to be propagated over long distances.
Many potential applications of high-power diode lasers, such as optical radar, satellite communications, laser printers, optical information storage and retrieval, and laser-to-fiber couplings, require a near-diffraction-limited single-lobed output beam which can be angle-scanned or switched.
It is thus desirable to integrate the beam scanning and switching mechanism with the laser source to reduce system complexity and size, and to increase reliability and stability. In the case of low-power narrow-stripe diode lasers, this has been done by incorporating a spatial phase controller into the laser cavity. For twin-stripe lasers, differential pumping has been used for beam scanning and switching. For high-power ten-stripe diode laser arrays, a separate on-chip master laser may control the array and provide beam scanning.
U.S. Pat. No. 5,003,550 by Welch et. al. discloses an invention that uses a low power (10-100 mW) narrow-stripe (5 micron wide) single-mode laser as a master oscillator. The lasing output from this master oscillator is subsequently acted on by amplifiers and phase controllers which are located external to the laser cavity. The sole function of the phase control array of Welch et. al. is to provide lateral beam steering of the amplified output beam, but it in no way does the array control the lasing behavior of the master oscillator laser. The current invention discloses a high-power broad-area laser with an internal phase controller that phase locks the lasing beam.
U.S. Pat. No. 4,995,048 by Kuindersma et. al. discloses a phase controller serving a very different purpose than the invention disclosed herein. The Kuindersma invention relates to narrow-stripe (few micron wide) lasers of the double channel planar buried heterostructure and buried heterostructure types which, because of the narrowness of their waveguiding region, operate in a single lateral mode. The phase control section in this type of laser counteracts the longitudinal variation in the refractive index in the Bragg reflector to prevent mode oscillation or mode jumping of the longitudinal lasing mode. Because the phase shift generated by the phase control section is uniform in the lateral extent of the laser, no beam steering or switching is possible. Additionally, the type of laser differs between the instant invention and the Kuindersma invention. For example, the current invention discloses mirrors created by cleaving facets of the semiconductor material, whereas the Kuindersma invention uses a distributed Bragg reflector at one end of the laser.
U.S. Pat. No. 4,965,806 by Ashby et. al. discloses a broad-area semiconductor laser diode that includes a structure for controllably varying a lateral refractive index profile of the diode to substantially compensate for junction heating effects during operation. The invention only uses thermal mode of operation to phase-lock the laser. The invention disclosed herein combines thermal mode with gain mode for additional flexibility in angle-switching and scanning.
U.S. Pat. No. 4,940,303 by Abeles et. al. discloses a phase control array located external to the laser, although both the laser and phase control array may be formed on the same substrate. The essential feature of the waveguide array phase controller of Abeles et. al. is the nonconstant, i.e., linearly increasing, spacing between adjacent parallel equal-length waveguide elements essential for reducing the side lobe intensity of the output beam. Additionally, separate electrical contacts and circuitry for controlling the phase retardation are required for each of the many (from 12 to 400) waveguides. Finally, a far-field beam detector is an essential part of the system for phase sensing and feedback to the phase control circuitry.
U.S. Pat. No. 4,903,275 by Ettenberg et. al. discloses a laser structure having an array of two or more regularly spaced waveguides, each with separate electrical contacts extending parallel to the waveguides over the entire length of the array. Under normal operating conditions, i.e., with identical electrical currents to each separate waveguide in the array, the array is phase-locked to produce a single-lobed output beam. When the electrical current to one or more of the waveguides is altered from this normal operating condition, the array becomes dephased, resulting in a multiple-lobed output beam. Beam steering is possible by varying the electrical current to each of the separate waveguides to control the nature and number of the far-field emission lobes. The type of semiconductor diode laser used is of the index-guided type instead of the gain-guided type disclosed herein.
U.S. Pat. No. 4,847,856 by Sugimura et. al. discloses a laser using fixed one-quarter-wavelength phase shifts in at least three locations. The phase shift is constant across the lateral extent of the laser and is built into the distributed-feedback semiconductor diode laser structure. Note that because the phase shift generated by the phase control section is uniform in the lateral extent of the laser, no beam steering or switching is possible. Moreover, the application of electrical current to the phase-shift sections in the Sugimura laser results in a modulation of the lasing frequency. Finally the laser of Sugimura et. al. uses a distributed-feedback type laser instead of a gain-guided laser as disclosed herein.
U.S. Pat. No. 4,803,686 by Brock discloses techniques for wavefront sensing for adaptive optics. A semiconductor diode laser is used with a narrow entrance aperture and only supports a single transverse mode of oscillation. The laser requires injection from a weak input probe beam having wavefront aberrations. The laser acts principally to spatially filter this input probe beam, to amplify the beam via a nonlinear four-wave mixing process, and to generate a uniphase reference output beam for interfering with the aberrated beam in order to detect the wavefront aberrations.
U.S. Pat. No. 4,751,705 by Hadley et. al. discloses a semiconductor diode laser having a single-lobed far-field emission beam which can be scanned in angle. However, it too differs from the invention disclosed herein. It discloses a laser using injection locking techniques to control the phase tilt across a semiconductor diode laser array to produce a single-lobed far-field emission beam, scannable in angle. This method requires a separate single-frequency wavelength-tunable laser source for injection locking the array. The injected radiation is confined to a single end-element of the array. The array emission beam is scanned in angle either by varying the frequency of the injected light or by varying the electrical current to the diode laser array. In the latter case, varying the electrical current will alter the output power level which may be undesirable for many applications. Also, the beam scanning by Hadley et. al. is uni-directional, being determined by which side of the array is injected.
U.S. Pat. No. 4,719,636 by Yamaguchi discloses a laser with a uniform phase shift across the lateral extent of the laser. The phase control section performs no role in phase-locking the lateral modes of the laser or in steering or switching the output beam angle. This phase control section is used to accomplish continuous wavelength tuning, free from longitudinal cavity mode hopping. The successful operation of this device requires that the electrical current to the phase control section be related to the electrical current to the grating wavelength-tuning section in such a manner that their ratio remains constant. Finally, the semiconductor diode laser is a distributed Bragg reflector type in the patent of Yamaguchi.
An article titled High Power, High Brightness 2W and 3W cw AlGaAs Laser Diode Arrays with Long Lifetimes by M. Sakamoto et. al. in Electronics Letters, Vol. 26, No. 11, dated May 24, 1990, discloses two types of AlGaAs single-quantum-well separate-confinement heterostructure (SQW-SCH) cw high power lasers. No discussion is made of any attempt to phase-lock the resultant laser beams. No discussion is made of beam scanning or switching. It is presumed that the laser emits in a broad twin-lobed far-field beam.
An article titled Tilted-Mirror Semiconductor Lasers by J. Salzman et. al. in the Appl. Phys. Lett. Vol. 47, No. 1, dated Jul. 1, 1985, discloses a broad-area GaAs heterostructure laser with a titled mirror. The tilted mirror was fabricated by etching, and the laser is operated in a smooth and stable single lateral mode with a high degree of spatial coherence. Construction of these lasers differ significantly from the construction of the lasers disclosed herein. For example, the mirror surfaces of the disclosed laser are created by cleaving the semiconductor material.
An article titled Modal Analysis of Semiconductor Lasers with Nonplanar Mirrors by J. Salzman et. al. in the IEEE Journal of Quantum Electronics, Vol. QE-22 No. 3, dated Mar. 3, 1986 discusses phase locking broad-area lasers with nonplanar mirrors. The disclosure is similar to that disclosed in the previous article by the same author disclosed herein.
An article titled Laser Beam Scanning Using a Local Deflector Integrated with an Effective Mode Filter by S. Mukai et. al. in Appl. Phys. Lett. Vol. 51 No. 25, dated Dec. 21, 1987 discloses semiconductor lasers where an injection-type deflector is combined with a long waveguide, resulting in an effective mode filter. The beam deflection mechanism of the device is prismlike when the deflector was short. The laser cavity consists of a beam-deflecting front part with twin nitride-stripe electrodes and a rear part with a very long mesa-striped waveguide. The invention disclosed herein only requires one electrode for beam scanning or switching.
An article titled Analysis of a Double-Heterostructure Spatial-Phase Controller for Diode-Laser Beam Steering by S. Mukai et. al. in the IEEE Journal of Quantum Electronics, Vol. 24, No. 12 (December 1988) is another disclosure of the lasers discussed in the previously referenced article by S. Mukai. The phase controlled by two electrodes in a front part. The rear part of the laser is a longer mode filter.
An article titled Beam Scanning and Switching Characteristics of Twin-Striped Lasers with a Reduced Stripe Spacing by S. Mukai et. al. in Optical and Quantum Electronics 17 (1985), pp. 431-434, discusses output-beam scanning and switching characteristics of lasers with separately controlled twin-stripe electrodes. The spacing between the two stripes is small, 5 microns from center to center. No integrated phase controller is shown, and the invention disclosed therein does not require twin-stripe electrodes.
An article titled Beam Scanning with Twin-Strip Injection Lasers by D. Scifres et. al. in Appl. Phys. Lett. Vol. 33 No. 8, dated Oct. 15, 1978, discloses angular scanning of the far-field intensity peak of a GaAs/GaAlAs double heterostructure (DH) injection laser. The scan is accomplished by adjusting the current levels between two closely-spaced stripe contacts, thereby creating an asymmetric gain/refractive index profile through which the laser beam propagates. The laser disclosed herein allows beam scanning and switching through the use of an integrated phase controller without the necessity of using a double stripe.
An article titled Integrated Injection-Locking High-Power Diode Laser Arrays by J. P. Hohimer et. al. in Appl. Phys. Lett. Vol. 55 No. 6, dated Aug. 7, 1989, discloses an integrated injection-locked high-power cw diode laser array with an on-chip independently controlled master laser. The device emits a near-diffraction-limited single-lobed far-field emission beam at single-facet powers up to 125 mW. Additionally, beam steering is accomplished by current tuning the emission wavelength of the master laser. The device integrates a master laser (ML) on the same chip as the slave array (SA) while minimizing feedback effects to the ML. The invention disclosed herein does not utilize a ML to phase lock the laser.
An article titled Injection-Locking Characteristics of Gain-Guided Diode Laser Arrays with an "On-Chip" Master Laser by J. P. Hohimer et. al. in Appl. Phys. Lett. Vol. 56 No. 16, dated Apr. 16, 1990, reported measurements of the laser disclosed in the authors' previously mentioned article.
An article titled Interelement Coupling in Gain-Guided Diode Laser Arrays by J. P. Hohimer et. al. in Appl. Phys. Lett. Vol. 48 No. 22, dated Jun. 2, 1986, discusses the use of single-channel injection locking to study the interelement coupling behavior in a gain-guided diode laser array. This differs from the invention disclosed herein where the phase controller is integrated with the active laser.